Electric filter



Patented Oct. 8, 1940 UNITED STATES PATENT OFFICE ELECTRIC FILTER Application January 25, 1939, Serial No. 252,794

9 Claims.

This invention relates to electric wave filters and particularly to crystal filters adapted to admit a frequency band of predetermined width and reject a predetermined frequency whose location with respect to the admitted fundamental frequency may be varied within wide limits. The invention finds great utility in the field of communications although it is to be understood that its use is not limited thereto.

As one example of the use of my invention is cited radio receivers operating in high carrier frequency ranges such as in the amateur bands in the United States. In such receivers, a wave band of about cycles is ample for signaling purposes. This has resulted in extremely close (in comparison to the carrier frequency) spacing between channels.

Obviously microscopic channel spacing requires extreme selectivity. In addition, it frequently becomes necessary to eliminate an interfering signal of unusual power as compared to the general signal background, which interfering signal may be but 100 or 200 cycles away from the desired fundamental signal frequency.

In order to meet this situation, the crystal filter described in United States Patent No. 2,054,757 to Lamb was devised. In this filter, means are provided for moving the anti-resonant peak of the crystal above or below its resonant frequency. This type of filter is used in an intermediate frequency stage of a superheterodyne circuit.

Thus in Fig. 3 of this patent, the crystal filter is shown both as a selectivity control filter, i. e., band width control, and also as a rejection filter. These two functions should be independent but in the Lamb system this is not the case. An interlocking between the two is found. When one control is exercised it tends to throw off the previous adjustment of the other control. This is particularly objectionable at high frequencies of the order of 1500 kilocycles. The difiiculty resides in the fact that the capacitance of the rejection part of the filter is effectively in parallel with the capacitance of the selectivity portion of the filter. Since these capacitances are additive, it is evident that a variation of either one causes the total to vary'and thus result in both controls having a functional interlocking.

In order to eliminate this interlocking of both variables this invention was devised. Not only have I eliminated the interlocking between the band width control and the rejection frequency control, but in addition I have found that a quicker and more flexible operation of the rejection control results. By my invention, the rejection frequency may be moved from any predetermined value on either side of the admitted fundamental frequency smoothly up to and as close to the fundamental as may be desired, the approach being asymptotical in nature. The resulting filter maintains a constant impedance looking in so that no reflections or reactions are possible on the feeding circuit.

In general I accomplish the above by associating with the rejection control condenser a second condenser across the crystal and electrically coupling the two condensers in a manner to main' tain a substantially constant capacity across the input terminals of the filter. b

The filter is of general application and may be used for all frequency bands.

Referring to the drawings, Fig. 1 shows a circuit diagram of the filter; Fig 1A is a modification; Fig. 2 shows curves indicating the relationship between phasing condenser adjustment and rejected frequency; Fig. 3 is a circuit in which the filter may be used; Fig. 4 shows the filter characteristics; Figs. 5, 6 and 7 are curves for a specific phasing condenser. 30

Referring to Fig. 1, input lines l0 and II are connected to filter input terminals l2 and I3. These terminals are adapted to be supplied with alternating currents of different frequencies and forming a band such as might be found in a modulated carrier. It is understood, of course, that any number of frequencies may be supplied since the filter attenuates those frequencies outside of the band which it is adapted to pass.

Between input terminals l2 and I 3 are four arms l5, I6, I! and ill of a filter bridge. At the junction of arms 15, I6 and I1, l8 respectively, there are output terminals 20 and 2| connected to an output circuit 22 and 22'. In arms l5 and I6 are variable condensers 23 and 24 adapted to be operated by one control. The two condensers 23 and 24 may really be considered as a single condenser split up into two series connected sec-' tions. Across the two condensers is connected a suitable inductance 25. Under certain conditions between output points 20 and 2|.

this inductance may be omitted as a discrete element and reliance had upon the inductance of associated elements.

Arm 1'! contains a phasing condenser section composed of a stator 21 and rotor 28. Arm [8 contains the rest of the phasing condenser composed of a stator '20 also cooperating with rotor 28. Physically, condensers 21-28 and 2829 may be separate. The only requirement is that the series capacitance 2'|28 and 28-29 remain constant over the operating range of the filter. This may be accomplished in various ways. Thus two condensers may have a suitably shaped cam drive to provide the displacement for satisfying this requirement. It is also possible to cut the plates of a variable condenser to a formula to satisfy this condition. This will be elaborated later.

Arm i! has in shunt to condenser 2T--28, a piezo-crystal 3| in a suitable holder. This crystal is cut to resonate at the desired fundamental admitted frequency of the filter. Arm [8 has a variable resistance 33 in shunt to condenser 2829.

Looking from one input terminal toward the other input terminal, it is clear that the reactance of the filter will be determined by the setting of condensers 23 and 24. Since phasing condensers 2120-29 have a constant capacity, it follows that the band width controlled by condensers 22-23 will not be altered by an adjustment of the phasing condensers. The crystal itself will behave like an extremely sharply tuned circuit.

Upon adjustment of phasing condensers 21- 2829, the phase of currents going along arm I! to output junction 2| will be varied with respect to currents going along arm 18. In other words, the balance of the bridge for output purposes is shifted. In this way, the rejected frequency going along arms I! and I8 toward point 2| cancels out. It is clear that as the balance is shifted, the anti-resonant characteristics of the opposing arms I! and I8 change. This is evident if we consider the reactance of the system In that case I! plus 15 and I8 plus l6 each have different reactances as phasing condensers 21-48-29 are adjusted.

In order to compensate for variations in power factor of crystal 3! at different frequencies variable resistance 33 is used. This resistance may be varied with change in phasing condensers 2128-29 to maintain the power factor of the filter uniform.

The variation is such that when the phasing condensers are at their extreme positions; i. e., one section at a maximum value and the other section at a minimum value to provide extreme bridge unbalance, the resistance is at a minimum value. The resistance then varies to its maximum value as the phasing condensers are adjusted to balance the bridge and, of course, goes to its minimum value as the bridge unbalances in the reverse manner. The exact value of the resistance is a function of the crystal characteristics, particularly its Q. Thus for crystals operating in the neighborhood of 1500 kilocycles, resistance 33 may vary from 50,000 ohms to 250,000 ohms. The precise values and the variation characteristics, however, may be determined by simple experiment and the above values are merely given for illustrative purposes. In'fact, resistance 33 may be fixed at some suitable value intermediate the normal maximum and minimum with gain in control simplicity and no great sacrifice of emciency.

In Fig. 1A, a modification is shown wherein resistance 33', for compensation of power factor variation in the crystal, is disposed directly in series with section 28-29 of the phasing condenser. The resistance is at its maximum value at extreme bridge unbalance and varies down to its minimum value upon bridge balance. The resistance in such case may vary from a few ohms up to about 500 ohms. Or a fixed resistance of some suitable intermediate value may be used.

Referring to condensers 21--2829, the condition is that the series capacitance in arms I! and [8 of the bridge be maintained at a substantially constant value in spite of adjustments. It is clear that the capacity of the crystal portion of the circuit is added to that of section 2'l28 of the phasing condenser.

From a mechanical angle, the simplest solution to a design of condensers 2|2829 is to use conventional gang variable air condensers such as are used in radio receiving sets but shape the plates in such a manner that the constant series capacitance relation will hold.

Let us assume that C1 and C2 are two variable capacitances under consideration. Let M and N be the minimum values for C1 and C: respectively.

As applied here, the minimum value would include the crystal capacitance as well.

Then

It should be clearly understood that C1 and C: represent the variable portion in the equation so that either one of these can vanish. Let us assume that Cl and C2 each vary from zero to maximum values C1 and C2 respectively. It is obvious that as one of these variables increases,

the other decreases. Hence, when Cl is a maximum 1 1 1 01+ M W K (1) Also, when C2 is a maximum 1 l l H or N 1 (2) Solving Equation 1 for N, we have C. K K M T= A M+ 01 K (3) Solving Equation 2 for N, we have I r I, I N Cz Irl-Irflf M'Cz It follows, therefore, that Equations 3 and 4 are equal to each other.

M+C"-K M-K For the sake of brevity let Then N=X:Y.

In the above equations, K is a constant whose value is a function of the maximum and minimum capacitances of the phasing condensers and may be a known arbitrary quantity. Similarly, C1 and C2 represent arbitrary values whose magnitudes, like that of K, is dependent upon the general circuit constants. Hence,

X=Y can be solved for M if K has a fixed value or can be solved for K if M has a fixed value. Then from N=X=Y the value of N can be determined.

Referring back to the basic equation Owl-M C2+N K and solving respectively for C1 and C2, we have KC2+KN+KMC2M-MN and as C1 and C2 respectively attain maximum values. From the above equations and C1+M and C2+N curves, it is possible to plot one curve involving C +M C +N and C2 as the two variables. in which ratio Since the manner varies with angular rotation of condenser shaft is known, the curves of capacity against rotation for C1 and C2 may be determined directly from the above mentioned curves.

In particular, for high frequency work it is desirable to reduce the fixed capacitances, such as crystal mounting and minimum capacity of variable condenser to as low a value as possible since the minimum values determine how closely the anti-resonant peak can approach the fundamental frequency. This is illustrated in Fig. 2 showing phasing condenser adjustment plotted against the anti-resonant frequency.

The vertical axis indicates an extreme phasing condenser setting. Thus the point marked C1 may indicate the maximum capacity for one section of the phasing condenser and minimum for the other While C2 may indicate the reverse condition; i. e. the phasing condenser at the other end of its range. The horizontal axis indicates the rejected frequency with the crystal resonant frequency as at the origin. It will be observed that the greater the phasing condenser unbalance, the nearer the rejected frequency will approach the crystal resonant frequency. Obviously, this gap can never be eliminated. But by reducing minimum capacities, as pointed out before, the approach of the rejected frequency to the crystal frequency can be made exceedingly close, of he order of 100 cycles to a crystal frequency of 1500 kilocycles, about .007%.

From the previous equations, it will be noted that the maximum values Ci and C2 need not necessarily be equal. In other words, the rejection curves need not necessarily approach the crystal frequency with the same curvature nor need they terminate at the same distance from the crystal frequency. Thus on one side, the rejection frequency may come up to 100 cycles away from the crystal frequency, while on the other side it may come closer to or end further away. The condenser plates are not necessarily of the same shape.

The filter may be disposed in a suitable amplifier stage and, as pointed out, is adapted to be used in a receiver. For convenience, it is shown in the intermediate stage of a superheterodyne circuit. It is obvious, however, that the filter may be used in any system and may be applied to cover any desired frequency band.

Thus in Fig. 3, a transformer is supplied with intermediate frequency currents of 1500 kilocycles, or any other desired frequency. It is understood, of course, that this intermediate frequency has added or subtracted thereto the modulating frequency. Transformer 50 has a secondary 5| which is shunted by a variable condenser 52. This condenser 52 is adapted to control the band width. Across condenser 52 are two series condensers 53 and 54 of fixed capacity. Obviously, condensers 52, 53 and 54 must all be treated as a lump capacity with regard to band width control. A ground 55 is taken at the junction of condensers 53 and 54 and this ground is one output terminal of the filter. Points 56 and 51 may be considered as the input terminals of the filter.

From point 56 a piezo-crystal element 58 extends to the other output terminal 60. In parallel to the crystal is a variable condenser having opposing electrodes 6| and 62. From point 57 to point 50 is a variable condenser having electrode 63 cooperating with electrode 62. As will be understood, condensers 6|, 62 and 63 operate in a manner to maintain a substantially constant series capacitance between points 56 and 51 through these two arms only. The relative proportions of these capacitances vary of course. Across condenser 6263 is a resistance 65 which may be variable if desired. In practice, however, the variation of resistance 65 for amateur reception is not important and satisfactory results are secured with a fixed resistance.

As condensers 6 l--62-63 are varied, the power factor of crystal 58 varies with change in frequency to which these two arms are anti-resonant. Hence, the power factor across condenser 6263 should vary similarly to maintain the bridge balance. In the absence of such power factor balance, the rejection peak amplitude is smaller than is possible with this filter.

The output of the bridge may conveniently be fed through a coupling condenser 61. The condenser 61 leads to a coil 68 and thence may be handled in conventional manners. Condenser 61 and coil 68 together form an output load. Preferably this load should not be too large in comparison to the bridge impedance range since the rejection characteristics Will be undesirably affected.

In Fig. 4 are curves showing the characteristics of the filter. Curve A shows the transmission efficiency of various frequencies through the bridge filter with the rejected frequency outside of the band. By manipulating condenser 52 or adjusting the capacitance due to condensers 52,53 and 54, the width of the band may be controlled. The minimum width is naturally the resonant frequency of the crystal and is only a theoretical possibility as the capacities and resistances in the bridge approach zero. Curve B shows the effect of the rejection control operating within the band limits. The crystal curve itself is extremely sharp as is well known. Since this rejection curve is never symmetrical with respect to the bridge resonance curve, curve B will always have a non-symmetrical outline. The height of the rejection curve is a function of the power factor of the crystal and adjacent phasing condenser arm.

The interposition of resistance 65 tends to balance the losses in both bridge arms and results in a higher rejection curve amplitude. In other words, the rejection is more perfect.

It is to be understood, of course, that the various constants and ranges of variables will be different for different frequency bands. Furthermore, even for the same filter, the shapes of the two phasing condensers have an infinite variation. Either section of the phasing condenser may be taken as a standard and as a result may be endowed with definite capacity change characteristics and this will determine the other section characteristics and shape in accordance with the equations previously given As a specific example, a filter for handling a fundamental frequency of 1560 kilocycles with a modulating frequency range of about 7000 cycles is herewith described.

From well-known bridge equations, it may be determined that the desired phasing range will result with a maximum capacitance variation of 50 micromicrofarads, hereinafter abbreviated to mmf. Thus C1:C2:50 mmf. The series ca.- pacity K of the two phase condenser bridge arms may be chosen to fit circuit conditions and for the frequency of 1560 kilocycles may advantageously be taken as 10 mmf. Then from Equation 5 we obtain the following:

500+ 10M 5004OM ill-P40 ill- 10 Solving for M, we obtain M:11.92 rnmf. Also from the same Equation 5 and By plotting the logarithm of the expression against degree rotation of the gang condenser shaft, the phasing curve, as in Fig. 2, will be essentially symmetrical. This curve, Fig. 5, is a straight line. It may be any other shape, if desired.

Referring back to Equation '7, it will be necessary to assign various arbitrary values to C1 between its limits and calculate the resulting values for C2.

0 5c. 1 32.33 2 23.60 4 14.96 2 it? 1 32 1.03 10 50 0 It is now possible to plot C1+M against C2+N, shown in Fig. '7, although the curve itself is not 15 necessary. It is interesting to note how one varies with the other.

C1+MI CH-N To actually determine the capacity curves A and B, a new table of C1+M C +N is made corresponding to the changes in C1 and C2 given above.

Going to the curve of Fig. 5, we find the percent 5 rotation of the condenser corresponding to the above values. Thus we know that the full condenser range or corresponds to .1925.

We may, therefore, make up the following table: 50

1+M P t C.+N 5523.3? 01 C1 Fig. 6 shows two curves. Curve A1 shows the variation of C1 against percent shaft rotation (a logarithmic function of U i-1V1 5 C +N Curve B1 shows C2 plotted against percent shaft rotation. It is understood, of course, that 100% shaft rotation means the entire range and may, for example, be 180 customary in gang con- 70 densers. Different angular ranges may be used as desired. From these curves the actual fabrication of condenser plates (or drive cams) may be made.

Since the capacity of an ordinary variable air condenser is a direct function of the opposed electrode areas, it is merely necessary to find the shape of plate which yields the desired capacity C1 or C2, as the case may be, for a definite rotation of the shaft.. Each capacity increment dC is equal to RPdG, an increment of area where R is theradius vector and dG is the rotational angular increment of one electrode plate. Since dG is known and since the expression dC= R dG is known, it is obvious that R may be determined for each condenser increment.

In the above example, the figures are accurate for commercial purposes. Referring now to Fig. 3, the circuit constants for a 1560 kilocycle bridge are given.

Part No. Constant .microhenrys. 80 nmicrcmicrofarads. 5-50 do l 140 do. 140 do crolicnrys. 8O micromicrofarads 10-100 65 .ohma. 50, 000-250, 000

The input coil for 5| may have an inductance of 80 microhenrys and is closely coupled thereto. The crystal, of course, has a resonant frequency of 1560 kilocycles.

Fig. '7 shows the capacity relationship between phasing condenser sections where K is 10 mmf. In this curve C1+M is plotted against C2+N. Because of the constants chosen, the curve is more or less symmetrical. However, this is not always true.

What is claimed is:

1. An electric wave filter of variable pass-band width having a piezo-crystal element for determining the minimum width of the frequency band passed by the filter, means for increasing the width of the band over a substantial range and means for suppressing a selected frequency, said two last named means being independent of each other whereby an adjustment of one means has substantially no effect on the operation of the other means.

2. A bridge type electric wave filter having a piezo-crystal in one arm for determining the minimum extent of the frequency spectrum passed by said. filter, reactances including capacitors in the arms of said bridge, an input circuit connected across two bridge points, an output circuit connected across the other two points, means for varying the reactance of two crystal free arms in series across the input points to vary the width of the band passed by said bridge and means for varying the reactance of each of the remaining two arms in a manner to maintain the total series reactance thereof constant to suppress any selected frequency except the resonant frequency of the'crystal, said two means being independently variable and without substantial effect on each others operation.

3 A bridge type electric wave filter having a piezo-crystal in one arm for determining the minimum extent of the admitted frequency spectrum, reactances including capacitors in the arms of said bridge, an input circuit connected across two bridge points, an output circuit connected across the remaining two bridge points, means for varying the reactance of two crystal free arms in series across the input to vary the width of the band passed, and means for varying the capacitative reactance of each of the remaining two arms in a manner to maintain the total series capacitance' thereof substantially constant to. suppress any selected frequency except thezresonant fre quency of the crystal, said two means" being. in dependently variable and without substantial effect on each others operation.

4. A bridge type electric wave filter comprising four arms, an input circuit connected to two points and. anoutput circuit connected to the remaining two points reactances including capacitors in each arm, a piezo-crystal in one arm in parallel to the capacitor to determine the minimum width of the frequency spectrum passed, means for varying the reactance of two crystal free arms in series across the input to adjust the width of the band, and means for varying the relative proportions of reactance in the remaining two arms while maintaining the series reactance substantially constant to suppress any desired frequency except the resonant frequency of the crystal, said two means being independently variable and without substantial effect on each others operation.

5. The bridge filter of claim 4, wherein said reactance varying means in the remaining arms comprises a pair of variable condensers having the characteristic of varying the relative capacitances of each of these two arms while maintaining at a substantially constant value the series capacitances thereof.

6. The filter system of claim 4, wherein a resistor is connected across the capacitor in series with the crystal containing arm to compensate for varying power factors in the crystal at various frequencies.

7. A bridge type of electric wave filter comprising an input circuit, a pair of bridge arms in series across said input circuit, said arms having inductive and capacitative reactance with the junction of said arms forming an output terminal, two additional arms in series across the input circuit with their junction forming an output terminal, a crystal in one of said additional arms, a predominantly capacitative reactive element in the other additional arm, a reactance element connected across said crystal and means for varying the reactance of each of said additional arms with respect to each other while maintaining the series reactance of said additional arms substantially constant, said means being independent of each other and having no substantial eifect on the operation of each other.

8. A bridge type electric wave filter having a piezo-crystal in one arm for determining the minimum extent of the admitted frequency spectrum, reactances including capacitors in the arms of said bridge, an input circuit connected across two bridge points, an output circuit connected across the remaining two bridge points, means for varying the reactances of two crystal free arms in series across the input to vary the width of the band passed, means for varying the capacitative reactance of each of the remaining two arms in a manner to maintain the total series capacitance thereof substantially constant to suppress any selected frequency except the resonant frequency of the crystal, said two means being independently variable and without substantial effect on each others operation and a resistance in series with the capacitative reactance in that one of the remaining two arms which is free of crystal, said resistance being adapted to compensate for power factor losses present in the crystal.

9. In combination, a two-section series connected gang condenser, each section having means for varying the capacitance thereof between fixed limits, and means for simultaneously and varying the capacitance of each section in an opposite sense in accordance with the following g, g= 5 equations:

K C,+ KN K M C1M MN where the letters represent the quantities set 5 C,+ N K forth in the specification.

DANA H. BACON. 

